JP5219171B2 - Spectroscopic method and spectroscopic apparatus - Google Patents

Description

  The present invention relates to a spectroscopic method and a spectroscopic apparatus for analyzing a chemical species adsorbed on a surface of a thin film or a substrate formed on the surface of a substrate with high sensitivity and measuring light absorption characteristics of a sample. The present invention also relates to a spectroscopic method and a spectroscopic device using an optical fiber.

  As a method of analyzing a thin film formed on the surface of a substrate or the like or a chemical species adsorbed on the surface of a substrate or the like, infrared spectroscopic analysis is widely used. This is based on the fact that each interatomic bond has a specific absorption wavelength (wave number) in the infrared region. When the sample can be used as it is or as a solution or the like and can be filled into a cell or sandwiched into a rock salt plate, Fourier transform infrared spectroscopy (FTIR) is commonly used. In recent years, the total reflection attenuation method (ATR) for measuring absorption of evanescent waves in the thin film near the interface by totally reflecting infrared light from the transparent substrate side of the thin film formed on the surface of the transparent substrate has been developed. It came to be used. According to ATR, the sensitivity is improved by about 30 times compared to the transmission type analysis such as FTIR.

  On the other hand, cavity ring-down spectroscopy (CRD) has been actively developed in recent years. In cavity ring-down spectroscopy, a cavity is formed by at least two mirrors, a substance to be inspected (sample) is introduced into the cavity, and a sample is prepared by using ring-down pulse light attenuated by light absorption of the sample in the cavity. Are spectroscopically analyzed. In cavity ring-down spectroscopy, an absorption coefficient at each wavelength of a sample is obtained by measuring an attenuation constant in attenuation of light intensity mainly due to light absorption, and the sample is identified and quantified. In addition, as shown in Patent Documents 3 and 4 below, pulse light is circulated through a loop fiber instead of a cavity, or pulse light is reciprocated through a straight fiber reflected at an end face, thereby improving ring-down characteristics of the pulse light. A method for obtaining absorption characteristics of a substance by measurement is known.

JP 2000-338037 A JP 2001-194299 A JP 2004-333337 A USP 6,842,548B2 USP 7,012,696 B2

However, when CRD analysis is performed using continuously output laser light, the laser is made to resonate in the cavity, and then the laser input is cut off to analyze the output from the cavity (ring-down light). Was taken.
On the other hand, in recent years, a soliton pulse light source having a very narrow pulse width and variable wavelength has been developed. Therefore, the inventors of the present application have completed a new CRD spectroscopic apparatus using this wavelength-tunable soliton pulse light source and the like.

  In addition, the pulsed light source used in conventional cavity ring-down spectroscopy is compatible with light sources and optical systems that change continuously in the near-infrared region, which is important for biological spectroscopy with wavelengths of 1 μm to 2 μm. It wasn't. A conventional light source has a pulse width of the order of ns, and there is a problem that a cavity length of 1 cm or less cannot be reduced because the pulses do not overlap. For example, at 5 ns, the cavity length is at least 15 cm, but at 500 fs, it becomes 150 μm. As described above, the pulse width is very effective for making a microcavity and thereby observing a high-speed response phenomenon. However, it does not correspond to such a wide wavelength width and miniaturization of the cavity.

  Further, in order to increase the S / N ratio of CRDS, methods using the optical heterodyne detection method described in Patent Document 1 and Patent Document 5 have been invented. However, the wavelength dependence of an ultrasonic modulator, a polarizing beam splitter, a wavelength plate, etc. has been invented. However, there is no element that supports a wide wavelength range such as 1 μm to 2 μm, and measurement with a high S / N was impossible over a wide wavelength range.

In any of the ring-down spectroscopy methods described above, the absorption characteristic of the sample is obtained from the attenuation characteristic of the ring-down pulse. Therefore, if there is nonlinearity in the absorption characteristic of the sample, an error is included in the measured value, and there is a problem that the accuracy of the absorption rate is not improved. In order to improve the measurement accuracy, a ring-type pulse measurement system requires a wide dynamic range with good linearity.
In addition, when there is optical attenuation in the measurement system, there is a problem that the sample cannot be measured unless the absorption rate of the sample is larger than the attenuation rate of the measurement system.
Therefore, the present inventors examined whether ring-down spectroscopy with improved measurement accuracy by measuring the absorption characteristics of the sample in a state where the amplitude of the light interacting with the sample was a constant value, The present invention has been completed.

In any of the ring-down spectroscopy methods described above, the light used is pulsed laser light, and in order to obtain the wavelength absorption characteristics of the sample, it is necessary to perform ring-down spectroscopy by changing the wavelength of the laser.
For this reason, it is necessary to use a tunable pulse laser as the light source.
The present inventors have examined whether ring-down spectroscopy can be performed without using a wavelength tunable pulse laser, and have completed the present invention.

An object of the present invention is to solve the above-mentioned problems, and a first object is to realize a cavity ring-down spectroscopic method and a spectroscopic device having a new configuration.
The second object is to set the amplification factor of the optical amplifying element in a state where the ring-down light of the light interacting with the sample is not attenuated, and to measure the absorption characteristic of the sample from the amplification factor and the attenuated light quantity. Is to do so.
The third object is to realize ring-down without using a wavelength tunable pulse laser.
A fourth object is to improve measurement accuracy.

According to the first aspect of the present invention, a pulsed laser beam is guided to an optical fiber that interacts with a sample whose light absorption characteristics are to be measured, and a ringdown pulsed light due to the sample's light absorption is output to the outside to attenuate the ringdown pulsed light. In the spectroscopic method characterized by measuring the absorption characteristics of the sample from the characteristics, the pulse laser beam is a laser beam having a broad spectrum, the optical fiber is a strong dispersion optical fiber, the pulse width of the ring-down pulse light is sequentially increased, and the wavelength The wavelength absorption characteristic is measured from the ring-down attenuation constant of the pulse train in the time train corresponding to the above.
The optical fiber may be formed in a loop shape, and light may be circulated through the loop-shaped optical fiber, or it may be formed in a linear shape so that light is reciprocated with both end surfaces as reflection surfaces. May be.

  As the laser light, for example, a broadband super continuum laser light may be used. As the excitation light source, an arbitrary laser such as a normal semiconductor laser, other solid-state laser, or gas laser can be used. The present invention utilizes the fact that the period of the ring-down pulse is different for each wavelength by using a laser beam having a wide spectrum and utilizing the fact that the phase velocity changes for each wavelength using a strong dispersion optical fiber. . The broadband super continuum laser beam is obtained as follows. A broadband supercontinuum laser having a continuous spectrum in the range of 1.25 μm to 1.95 μm when an ultrashort pulse width (for example, soliton laser beam) laser light is incident on a dispersion-shifted highly nonlinear fiber having a length of about 5 m. Light is obtained. In addition, using a 200m dispersion-shifted highly nonlinear fiber, a broadband supercontinuum laser beam having a continuous spectrum in the range of 1.0 to 2.2 μm can be obtained (Norihiko Nishizawa, Toshio Goto, Solid State Physics Vol.39 No .10, (2004), pp665-678).

According to a second aspect of the present invention, there is provided a spectroscopic device for measuring a light absorption characteristic of a sample, a strongly dispersed optical fiber for guiding the laser pulse light to the sample whose light absorption characteristic is to be measured, and a laser pulse light having a wide spectrum width. And the pulse width of the ring-down pulse light that circulates or reciprocates through the optical fiber and outputs it to the outside, and in the attenuation characteristic of this ring-down pulse light, the pulse train in the time train corresponding to the wavelength And a processing device for obtaining a wavelength absorption characteristic from the ring-down attenuation constant of the spectroscopic device.
As the laser beam, for example, a broadband supercontinuum laser beam may be used as described above.

  The invention according to claim 3 includes a first optical transmission line optically coupled to the optical fiber, and a directional optical coupling element optically coupling the first optical transmission line to the optical fiber, and the first optical transmission. 3. The spectroscopic apparatus according to claim 2, wherein a laser device is provided at one end of the path, and a light receiving element of a processing device that receives ring-down pulse light is provided at the other end.

According to a third aspect of the present invention, there is provided a second optical transmission line in which ring-down pulse light is different from the first optical transmission line by another optical coupling element disposed at a position different from the directional optical coupling element in the optical fiber. The spectroscopic device according to claim 2, wherein a light receiving element of a processing device is connected to the second optical transmission line.
The present invention is characterized in that a system for introducing laser light and a system for deriving ring-down pulse light are separated.

Apart from the invention of the above claims, the following invention is also described in this specification.
First, in a spectroscopic device that measures the light absorption characteristics of a sample, an optical fiber that guides light to the sample whose light absorption characteristics are to be measured, a first optical transmission line that optically couples with the optical fiber and propagates pulsed light, and an optical fiber It consists of a processing device that outputs the ring-down pulse light that circulates or reciprocates to the outside and detects and processes this ring-down pulse light, and a plurality of optical transmission lines having different optical path lengths. And a second optical transmission line capable of switching the optical path length from the branching point to the input to the processing device in units of optical path lengths of the optical fiber.

  That is, the present invention switches a plurality of optical transmission lines so that the pulsed light is transmitted through the second transmission line and reaches the processing device at the same time as each ring-down pulse reaches the processing device. The feature is that

The second is a spectroscopic device characterized in that the second optical transmission line is composed of transmission lines having different optical path lengths by the number of ring-down pulses to be measured for one pulsed light. . In order to synchronize with all the ring-down pulses to be measured, transmission lines having different numbers of optical path lengths are required.
The third is a spectroscopic device characterized in that each of the plurality of optical transmission lines constituting the second optical transmission line is composed of an optical fiber wound around a piezo tube scanner.
An optical fiber is wound around the piezo tube scanner to form an optical transmission line, and the piezo tube scanner is vibrated to vibrate the effective optical path length, thereby causing the pulsed light propagated through the ring-down pulse and the second optical transmission line. Can be synchronously detected in the processing device.

The fourth is a spectroscopic device characterized in that the pulsed light is short pulsed light having a pulse width of 1 ps or less. It is also effective to use ultrashort pulse light having a pulse width of 500 fs or less.
The fifth is a spectroscopic device characterized by using ultrashort pulsed light from a near-infrared wavelength variable soliton pulsed light generating light source using a femtosecond laser.
The sixth is a cavity ring-down spectroscopic device characterized by using ultrashort pulsed light from a broadband supercontinuum light generating light source.

  Seventh, a signal light that is a pulse train of an ultrashort optical pulse having a predetermined wavelength and each pulse having a width of 1 ps or less and a reference light branched from the signal light are used to measure the reference light during measurement. A cavity ring-down spectroscopic analysis method characterized in that the optical path length is sequentially increased, and homodyne detection is performed on interference between ring-down light and signal light having passed through the cavity. The width of each pulse may be 500 fs or less.

  The eighth is characterized in that a pulse train of ultrashort light pulses by a near-infrared wavelength variable soliton pulse light generation light source by a femtosecond laser is used. Ninth, a pulse train of ultrashort light pulses from a broadband supercontinuum light generating light source is used as the light source.

  Tenth, a light source that generates an ultrashort optical pulse train having a predetermined wavelength and each pulse width of 1 ps or less, an optical branching device that branches the pulse train output from the light source into two paths, and the optical branching device A cavity having two high-reflecting mirrors connected to the first optical output of the first optical output, and a sample placed in an optical path formed by the two high-reflecting mirrors; and the second optical output of the optical branching means A cavity having a variable length optical path portion that is variable in optical path length, and homodyne detection means that outputs an interference between the optical output of the cavity and the optical output of the variable length optical path portion as an electrical signal. It is a ring-down spectroscopic analyzer. The width of each pulse may be 500 fs or less.

  The eleventh aspect is characterized in that the variable length optical path portion has a movable mirror that can vibrate and change its position.

The twelfth aspect is a spectroscopic method for measuring light absorption characteristics of a sample by propagating light to an optical fiber that guides light to a sample whose light absorption characteristics are to be measured, wherein an amplification element for amplifying light is provided in the optical fiber. This is a spectroscopic method.
In the thirteenth aspect, the light is step-wise decreasing light or pulsed light, and the amplification factor of the amplification element is set to an amplification factor that does not attenuate the ring-down light when the sample is not installed. The spectroscopic method is characterized in that a sample is placed, ring-down light is measured, and the absorption rate of the sample is measured from the attenuation characteristic of the ring-down light.

  In the fourteenth aspect, the light is light that decreases stepwise or pulsed light, and the sample is set to measure ringdown light, and the amplification factor of the amplifying element is set so that the ringdown light is not attenuated. It is a spectroscopic method characterized by controlling and measuring the absorption rate of a sample from this amplification factor.

  In the above three methods, the optical fiber may be formed in a loop shape, and light may be circulated through the loop-shaped optical fiber. Alternatively, the optical fiber may be formed in a linear shape, and both end faces thereof may be reflecting surfaces. The light may be reciprocated.

  A laser or LED light source can be used for the light. As the laser, any laser such as a normal semiconductor laser, other solid-state laser, or gas laser can be used. By using a wavelength tunable laser, the wavelength absorption characteristics of the sample can be measured. In addition, when a broadband supercontinuum laser is used, the wavelength absorption characteristic can be obtained by wavelength analysis of ring-down light received by the light receiving element.

  The light may be continuous light, light that decreases stepwise, or pulsed light. The light that decreases stepwise may be realized by blocking the light output from the light source itself, or may be realized by outputting a pulse. Further, a method of decreasing the optical coupling of the optical fiber with respect to the light output from the light source in a stepwise manner or a method of increasing the coupling in a pulsed manner can be used. When the optical coupling of the optical fiber is decreased stepwise, it means that the amplitude of light introduced into the optical fiber is decreased stepwise.

  When optically coupled in a pulsed manner, it means that the optical coupling is performed only for a short pulse period. Therefore, the amplitude of the light incident on the optical fiber becomes a step decreasing function or a pulse function. Further, by controlling the polarization direction of the light output from the light source, the light propagating through the optical fiber can be made into a step reduction function and a pulse function. When changing in a stepwise manner, the polarization direction is changed from a steep direction to another direction. When changing in a pulse manner, the polarization direction is changed in a steep direction, and the original direction is changed. It means changing to the polarization direction or another polarization direction.

Fifteenth, in a spectroscopic device for measuring the light absorption characteristics of a sample, an optical fiber for causing the sample to measure the light absorption characteristics to interact with light, an amplification element for amplifying the light, and the optical fiber are propagated. It is a spectroscopic device comprising a detection element that detects the intensity of light, and a processing apparatus that calculates the absorption characteristics of the sample by controlling the amplification factor of the amplification element according to the intensity of the light detected by the detection element.
According to the present invention, an amplifying element for amplifying light is provided in an optical fiber that interacts with a sample, and the absorption characteristic of the sample is measured by setting the amplification factor of the amplifying element according to the intensity of light propagating through the optical fiber. The feature is that

  In the sixteenth aspect, the light is light that decreases stepwise or pulse light, and the processing apparatus uses the amplification factor of the amplification element as the amplification factor that does not attenuate the ring-down light of the light when the sample is not installed. The spectroscopic apparatus is characterized in that the sample is set, the ring-down light is measured, and the absorption rate of the sample is measured from the attenuation characteristic of the ring-down light. This corresponds to the thirteenth method invention.

  In the seventeenth aspect, the light is light that decreases stepwise or pulsed light, and the processing apparatus measures the ring-down light by installing a sample, and an amplification element so that the ring-down light is not attenuated The spectroscopic apparatus is characterized in that the absorptance of the sample is set and the absorption rate of the sample is measured from the amplification factor. This corresponds to the fourteenth method invention.

  In the eighteenth aspect, the light is continuous light, and the processing apparatus controls the amplification factor of the amplifying element so that the amplitude of the light becomes a predetermined value, and measures the absorption characteristics of the sample from this amplification factor or loops. This is a spectroscopic device characterized in that the loss in the optical fiber cavity is reduced to 0, a sample is placed, and the absorption characteristics are measured from the amount of attenuated light.

Even if the light is continuous light, the absorption characteristics of the sample can be obtained. The amplitude of the light that circulates or reciprocates through the optical fiber converges to a value at which the amount of incident light on the optical fiber, the amount of loss due to sample absorption, and the amount of loss of other optical fiber systems become balanced. Since there is no absorption by the sample if the sample is not present, the amplitude of light propagating through the optical fiber is increased compared to the case where the sample is present. Therefore, by using this difference and controlling the amplification factor so that the amplitude of the light propagating through the optical fiber becomes a predetermined value, the absorption characteristic of the sample can be measured from this amplification factor.
Similarly, when the sample does not exist, the light amplification amount is set so that the loss amount of the optical fiber system becomes 0, and the absorption characteristic can be measured from the amount of attenuation when the sample exists.

  In the method invention of claim 1 and the apparatus invention of claim 2, a laser beam having a wide spectral width is used, and a strong dispersion fiber is used for the optical fiber. Since the phase velocity of the strongly dispersed optical fiber changes with the wavelength, the period of the ring-down pulse light changes with the wavelength. Therefore, the ring-down pulsed light has a longer propagation distance through the optical fiber as the number of ring-downs increases, so the time width of the pulsed laser light increases. From the attenuation characteristic of the ring-down pulse light train, the ring-down attenuation coefficient of the pulse train corresponding to the time train corresponding to the wavelength can be obtained, and the attenuation coefficient at each wavelength can be obtained. Thereby, the wavelength absorption characteristic can be obtained at a time, and the sample can be identified and analyzed.

  According to the third aspect of the present invention, the incidence of the pulsed laser beam on the optical fiber and the extraction of the ring-down pulse from the optical fiber are realized by one directional optical coupling element. Therefore, the device configuration is simplified.

  In the device invention of claim 4, since the incidence of the pulse laser beam to the optical fiber and the extraction of the ring-down pulse are performed by separate directional optical coupling elements, the pulse laser beam output from the laser device is Since it does not enter the light receiving element, it is possible to detect the ring-down pulse with high accuracy.

Apart from the claimed invention, the invention described in this specification has the following effects.
In the first invention, the pulsed light propagating through the first optical transmission line is partially branched and input to the optical fiber. In this optical fiber, the pulsed light is sequentially attenuated by light absorption by the sample, and ring-down pulsed light is obtained. The ring-down pulse light is sequentially output from the optical fiber to the outside and is input to the processing apparatus. On the other hand, the pulse light propagating through the first optical transmission line is branched and then propagated through one selected optical transmission line of the second optical transmission lines and input to the processing device. At this time, in the processing apparatus, a certain ring-down pulse light and the original pulse light arrive at the same time. As a result, synchronous detection of the ring-down pulse light becomes possible. If the optical transmission path constituting the second optical transmission path is changed, the ring-down pulse light that has circulated for a time corresponding to the optical path length of the optical transmission path and the original pulse light are simultaneously input to the processing device, and the ring Synchronous detection of down-pulse light is possible.

In the second invention, a predetermined number of ring-down pulse lights can be synchronously detected by sequentially selecting each transmission line constituting the second optical transmission line in synchronization with the pulse light.
In the third invention, the optical transmission path constituting the second optical transmission path is composed of an optical fiber wound around a piezo tube scanner, so that the piezo tube scanner is expanded and contracted to expand and contract the optical fiber, and the optical path is electrically The length can be changed. By electrically vibrating the diameter of the piezo tube scanner, the optical path length can be oscillated with a certain width around a certain length. In the vibration width of the optical path length, there can be an optical path length for causing the ring-down pulsed light reaching the processing apparatus and the original pulsed light to reach simultaneously.

In the fourth invention, when short pulse light having a pulse width of 1 ps or less is used, high sensitivity cavity ring-down spectroscopy by synchronous detection is possible even if the optical fiber length is short. It is further desirable to use ultrashort pulse light of 500 fs or less. In addition, since the width of the pulse for synchronous detection is narrow, the measurement accuracy is improved and the apparatus can be miniaturized. Such a system is particularly effective for microanalysis, such as qualitative and quantitative analysis of thin films, identification of chemical species for surface adsorption, and the like.
The fifth invention also has the same effect as the 44th invention.
In the sixth invention, the wavelength absorption characteristic of the sample can be measured by performing wavelength analysis in the processing apparatus.

  When an ultrashort light pulse with a pulse width of 1 ps or less is used, highly sensitive cavity ring-down spectroscopic analysis by homodyne detection is possible even in a cavity with a very short cavity length. Such a system is particularly effective for microanalysis, for example, qualitative and quantitative analysis of thin films, identification of chemical species for surface adsorption, etc. (Inventions 7 to 11). The pulse width may be 500 fs or less.

  An optical pulse train is branched into two optical paths, one is guided to a cavity, the other is guided to an optical path having a movable mirror whose optical path length is variable, an optical pulse train of the optical path of the variable optical path length, and a pulse train of ring-down light by the cavity The homodyne detection of the interference. By gradually increasing the optical path length on the variable side so that there is a pulse that matches the optical path length of the ring-down light, the intensity of the ring-down light can be sequentially detected from a plurality of pulses. If the movable mirror can be oscillated and the movable mirror is moved on it, a number of timings that "match the optical path length of the ring-down light" can be provided, making initial adjustment easy and ensuring that each ring-down light is reliably transmitted. It becomes possible to perform homodyne detection.

  In the twelfth method invention and the fifteenth device invention, an amplifying element for amplifying the light is provided in the optical fiber that guides the light to the sample. Therefore, the sample absorptance can be measured in a state where the loss of the measurement system and the loss of the sample are compensated. Therefore, highly accurate measurement is possible.

  In the thirteenth method invention and the sixteenth device invention, the light is reduced in a stepwise manner or pulsed light. Therefore, the attenuation characteristic of this ring-down light can be used for measurement. The amplification factor of the amplifying element is set to an amplification factor that does not attenuate the light ring-down light when the sample is not installed, so that the loss of the measurement system is compensated, and the sample absorption rate is measured in the compensated state. Therefore, the measurement accuracy of the sample absorption rate is improved. That is, in the present invention, first, the attenuation characteristic of the ring-down light resulting from the absorption of only the sample is measured with the loss of the measurement system compensated, and the absorption characteristic of the sample is measured from the attenuation characteristic. .

In the fourteenth method invention and the seventeenth device invention, the sample is set and the ring-down light is measured, and the amplification factor of the amplifying element is controlled so that the ring-down light is not attenuated. It is characterized by measuring the absorption rate.
That is, light is ring-down due to light absorption of the sample, but the amplification factor of the amplifying element is feedback-controlled so as not to cause this ring-down, and the absorption coefficient of the sample is measured from this amplification factor. Therefore, since the light interacting with the sample always has a constant amplitude, the absorption characteristic at a predetermined light intensity can be obtained. That is, the nonlinear characteristic of the absorption characteristic can be measured. In addition, since the light amplitude is constant, there are few errors in the measurement value, and the measurement accuracy is improved.

  In the eighteenth aspect of the invention, the light is continuous light, and the processing device controls the amplification factor of the amplification element so that the amplitude of the light becomes a predetermined value, and the amplification factor or the attenuation light amount of the ring-down light. The absorption characteristics of the sample are measured. Even in this case, it becomes possible to compensate for the loss of the measurement system and to measure the absorptance of the sample with a constant light intensity, thereby improving the measurement accuracy.

The 1st graph for demonstrating the content of the synchronous detection in the Example of this invention. The 2nd graph figure for demonstrating the content of the synchronous detection in the Example of this invention. The 3rd graph for demonstrating the content of the synchronous detection in the Example of this invention. The 4th graph for demonstrating the content of the synchronous detection in the Example of this invention. The 5th graph for demonstrating the content of the synchronous detection in the Example of this invention. The block diagram which shows the structure of the ring-down spectroscopy apparatus which concerns on one specific Example of this invention. The block diagram which shows the structure of the ringdown spectroscopy apparatus which concerns on the specific Example 2 of this invention. The measurement figure which shows the interference waveform when the delay path is set to 3 m and 30 m using the spectrometer of Example 2, and the absorption rate of air and methanol is measured. The measurement figure which showed the characteristic with respect to the methanol density | concentration of the absorption rate when the absorption rate of methanol was measured by changing the concentration with the spectroscopic device of Example 2. The block diagram which shows the structure of the cavity ringdown spectroscopy analyzer 300 which concerns on the specific Example 3 of this invention. The block diagram which shows three methods of arrange | positioning the board | substrate with which the sample was adhered to the surface in the cavity area | region. The characteristic view which shows the interference waveform of the laser obtained by the apparatus of Example 3 shown in FIG. FIG. 6 is a characteristic diagram showing a spectrum of supercontinuum light used in the apparatus of Example 3. FIG. 10 is a characteristic diagram illustrating a laser interference waveform when supercontinuum light having the spectrum illustrated in FIG. 9 is used in the apparatus according to the third embodiment. The block diagram which showed the apparatus which concerns on the specific Example 4 of this invention. The block diagram which showed the apparatus which concerns on the specific Example 5 of this invention. FIG. 10 is a characteristic diagram showing the results of measuring the amplification factor of the semiconductor optical amplifier when the current injected to the power source of the semiconductor optical amplifier used in the apparatus of Example 5 was changed. FIG. 10 is a characteristic diagram illustrating the reason why a long optical delay optical path cannot be used when a semiconductor optical amplifier is not used in the apparatus according to the fifth embodiment. FIG. 10 is a characteristic diagram showing that the interference intensity with respect to the optical delay optical path length is improved by inserting a semiconductor optical amplifier and amplifying light in the apparatus of the fifth embodiment. Explanatory drawing which showed that the optical delay optical path length which can be used in order to improve the measurement precision of an absorption coefficient can be expanded by inserting a semiconductor optical amplifier in the apparatus of Example 5, and amplifying light. The block diagram which showed the apparatus which concerns on the specific Example 6 of this invention. FIG. 10 is a characteristic diagram showing an interference waveform of a laser obtained by the apparatus of Example 6. (A) shows the case where there is no absorption by the sample, and (b) shows the case where absorption by the sample exists. The block diagram which showed the apparatus which concerns on specific Example 7 of this invention. The block diagram which showed the apparatus which concerns on the specific Example 8 of this invention. The block diagram which showed the apparatus concerning the specific Example 9 of this invention. The block diagram which showed the apparatus concerning the specific Example 12 of this invention. The wave form diagram which showed the ringdown pulse waveform for demonstrating the operation | movement in the apparatus of Example 12. FIG. The characteristic view which showed the relationship between the wavelength and propagation delay time in the optical fiber which has a strong dispersion characteristic. The block diagram which showed the apparatus concerning the specific Example 13 of this invention. The block diagram which showed the apparatus concerning the specific Example 14 of this invention.

  Hereinafter, specific examples of the present invention will be described with reference to the drawings. In addition, this invention is not limited to a following example. First, synchronous detection in the present embodiment will be described.

Embodiment 1
First, pulse light is circulated through a looped optical fiber. The optical fiber and the sample interact with each other, and the pulsed light is sequentially attenuated by light absorption by the sample to obtain ring-down pulsed light (pulse train), which is input to an external processing apparatus. If the optical path length of the loop-shaped optical fiber is L, the difference in optical path length of each ring-down pulse light is L. If the speed of light is c, the time interval between adjacent ring-down pulse lights is L / c. It becomes. That is, FIG. As shown in A, the pulsed light input to the processing apparatus includes transmitted light S ₁ , S ₂ , S ₃ , S ₄ ,... That directly enter without circulating through the optical fiber. In addition, after the time L / c of the transmitted light S ₁ , the processing device further receives a one-time ring-down pulsed light L ₁₁ that is optically output through the looped optical fiber, and further the time of the pulsed light L ₁₁ . After L / c, the two-time ring-down pulsed light L ₁₂ that is optically output through two rounds of the optical fiber is sequentially input. As described above, the ring-down pulse light is sequentially output from the optical fiber at time L / c intervals, and is sequentially input to the processing device. The same applies to the transmitted light S ₂ , S ₃ , S ₄ and their ring-down pulse light. By adjusting the optical path length L and the pulse interval T _p of the optical fiber, the ring-down pulse following one transmitted light. It is easy to prevent the light set from overlapping the next transmitted light. Hereinafter, transmitted light (pulsed light that is not absorbed by the sample) and ring-down pulsed light are referred to as signal light.

On the other hand, the pulse light propagates through the second optical transmission line, which is a different path from the loop-shaped optical fiber, and enters the processing apparatus. Hereinafter, this pulsed light is referred to as reference light. Using this reference light, ring-down pulse light including transmitted light is synchronously detected. As a specific example, a detection method based on the correlation between signal light and reference light, such as homodyne detection and differential detection, can be used. Assuming that the optical path length of the reference light propagation path is completely matched with the optical path length of the transmitted light propagation path, the synchronous detection of the reference light and the transmitted light is the product of the amplitude of the transmitted light and the amplitude of the reference light. Becomes a pulse having an amplitude of. The detection output at this time is A. Next, assuming that the optical path length of the propagation path of the reference light is exactly the same as the optical path length of the propagation path of the ring-down light once, the synchronous detection of the reference light and the signal light is the product of the reference light and the signal light, That is, a pulse in which the amplitude of the reference light is modulated by the amplitude of the signal light is obtained. At this time, the detection output is αA (0 <α <1). Similarly, assuming that the optical path length of the reference light is completely coincident with the optical path length of the n-down ring light, the detection output is α ⁿ A (0 <α <1).

  However, it is not always easy to completely match the optical path length of the propagation path of the reference light with the optical path length of the propagation path of the transmitted light and each round-down pulse light. This is a difficulty that comes from using signal light with a short pulse width. Therefore, an optical transmission path such as an optical fiber wound around a piezo tube scanner is provided in the propagation path of the reference light so that its optical path length can be changed in terms of vibration with respect to the reference optical path length. Then, due to the vibration of the piezo tube scanner, adjacent pulses of the reference light pass through different optical path lengths, and the pulse interval of the reference light is in a direction in which the optical path length becomes longer than the pulse interval at the standard optical path length. In the process of fluctuation, the pulse interval becomes longer, and conversely, in the process of fluctuation in the direction in which the optical path length becomes shorter, the pulse interval becomes shorter. At this time, if the vibration amplitude a of the optical path length is shorter than the optical path length L of the optical fiber, the processing of the pulsed light due to the change in the pulse interval of the reference light rather than the time interval L / c between the adjacent ring-down pulsed lights. The change a / c in the input time to the apparatus can be reduced. In this way, the optical path length of the reference light propagation path branched from the signal light having a desired pulse frequency (pulse interval) is oscillated with respect to the reference optical path length, so that the optical path length of the reference light is changed to the transmitted light. In addition, it is possible to easily achieve almost perfect matching with the optical path length of the propagation path of each ring-down pulse light.

FIG. Below B, specific numerical values will be exemplified to describe the synchronous detection of two pulse trains in the present embodiment. A piezo tube scanner for the second optical transmission line through which the reference light propagates with the signal light as pulse light having a pulse width of 100 fs (1 × 10 ⁻¹³ seconds) and a pulse interval of 2 × 10 ⁻⁸ seconds (pulse frequency 50 MHz). Is set to 80 Hz. In addition, in the reference optical path length of the second optical transmission path, the optical path length of the reference light matches the optical path length of the signal light. The vibration amplitude of the second optical transmission path is assumed to vibrate ± 2 mm as the optical path length.

The time function of the optical path length x is x = 2 sin 160πt + x ₀ . x ₀ is the reference optical path length. The change speed v (mm / sec) of the optical path length is v = 320πcos 160πt, and when considering near the center of simple vibration, v = 1000 mm / sec at t = 0. Then, the optical path length changes by 2 × 10 ⁻⁸ m within the reference light pulse interval of 2 × 10 ⁻⁸ seconds. This means that the pulse interval of the reference light propagating through the second optical transmission line is longer or shorter by 6.67 × 10 ⁻¹⁷ seconds.

Then, for example, in the processing apparatus, the 0th pulse is generated by the signal light and the reference light that has passed through the second optical transmission line as shown in FIG. Like B, the pulse width is shifted by 1 × 10 ⁻¹³ seconds, the reference light pulse time is earlier, and the reference light pulse interval is 6.67 × 10 ⁻¹⁷ than the signal light pulse interval. If the second is longer, 1 × 10 ⁻¹³ seconds ÷ 6.67 × 10 ⁻¹⁷ seconds = 1500, so the pulse of the reference light coincides with the pulse of the signal light at the 1500th pulse, and the signal light at the 3000th pulse. The reference light is shifted by exactly 1 × 10 ⁻¹³ seconds of the pulse width, and the time of the reference light pulse is later. Since the pulse interval of the signal light is 2 × 10 ⁻⁸ seconds, the time interval between the 0th pulse and the 3000th pulse is 6 × 10 ⁻⁵ seconds. Note that the change in the optical path length during this period is 60 μm.

When such signal light and reference light interfere with each other and perform synchronous detection, FIG. An output such as C is obtained. FIG. Interference occurs between the signal light and the reference light during 3000 pulses in the state B, and is 0 at the 0th pulse, maximum at the 1500th pulse, and exactly 0 at the 3000th pulse. 2999 peaks are obtained below the C envelope (indicated by the dotted line). This envelope and the 2999 peaks below it exist for 6 × 10 ⁻⁵ seconds and do not exist until 0.025 seconds after the optical path length of the second optical transmission line returns to the original length. That is, the envelope and the 3000 peaks below it exist for 6 × 10 ⁻⁵ seconds at intervals of 0.025 seconds, and do not exist at other times.

When viewed macroscopically, very sharp peaks with a width of 6 × 10 ⁻⁵ seconds will occur at 0.0125 second intervals (FIG. 1.D). The pulse waveform is as shown in FIG. Even if it is not a square wave like B, it can be said similarly in a considerably wide range from the center of simple vibration.
In addition, FIG. Regarding the difference in interference between the B 1500th pulse and the preceding and following pulses, for example, 18.4 waves exist in one pulse at a wavelength of 1.63 μm, and the phase difference is as small as 2π / 80. Then, interference between a plurality of sets of signal light and reference light in the vicinity of the peak of the envelope produces an intensity substantially equal to the intensity of the peak of the envelope, so that the pulse phases of the signal light and the reference light are completely matched. There is no need to make adjustments. Further, it is not necessary to make the reference optical path length of each optical transmission path constituting the second optical transmission path exactly equal to the optical path length of each ring-down pulse light. , ± 2 mm) or less may be present. That is, it is sufficient that the optical path length of the ring-down pulse light and the optical path length of the reference light coincide with each other in a range where the optical path length vibrates.

Therefore, the reference optical path length of the 0th optical transmission line of the second optical transmission line is as shown in FIG. The optical path length of the transmitted light S ₁ , S ₂ , S ₃ , S ₄ ,... Of A may be matched with an error corresponding to the vibration amplitude of the optical path length. Similarly, the reference optical path length of the first optical transmission line of the second optical transmission line is as shown in FIG. The optical path length of the one-time ring-down pulsed light L ₁₁ , L ₂₁ , L ₃₁ , L ₄₁ ,... Similarly, the reference optical path length of the second optical transmission line of the second optical transmission line is as shown in FIG. The optical path lengths of the two ring-down pulse lights L ₁₂ , L ₂₂ , L ₃₂ , L ₄₂ ,... Similarly, the reference optical path length of the nth optical transmission line of the second optical transmission line is as shown in FIG. The optical path lengths of the n ring-down lights L _1n , L _2n , L _3n , L _4n ,... Of A may be matched with an error corresponding to the vibration amplitude of the optical path length. Thus, when the optical path length of each optical transmission line constituting the second optical transmission line is designed, FIG. As in E, synchronous detection between the transmitted light and the reference light propagated through the 0th optical transmission line, synchronous detection between the one-time ring-down pulse light and the reference light propagated through the first optical transmission line, Synchronous detection of the ring-down pulse light and the reference light propagated through the second optical transmission line, ..., synchronous detection of the reference light propagated through the n-th optical transmission line of the n-time ring-down pulse light and Each of these becomes a group of extremely sharp pulses having a width of 6 × 10 ⁻⁵ seconds.

  FIG. 2 is a block diagram showing a configuration of a ring-down spectroscopic device 100 using a loop-shaped optical fiber having a length of about 150 m according to a specific embodiment of the present invention. The spectroscopic device 100 includes a first optical transmission line 4 including a wavelength-tunable femtosecond soliton pulse light source 1, a half-wave plate 2, a spherical lens 3a, and an optical fiber. The first optical transmission line 4 is coupled to a loop-shaped optical fiber 6 via an optical switch 5. A sample 10 that interacts with pulsed light propagating through the optical fiber 6 is coupled to the loop-shaped optical fiber 6. The optical fiber 6 is provided with an optical amplifier 7 in order to make the attenuation constant of the ring-down pulse light due to light absorption of the sample 10 appropriate. The optical amplifier 7 may be omitted, but by using this to appropriately amplify the ring-down pulse light, it is possible to measure the light absorption characteristics of the sample with high accuracy and sensitivity. The first optical transmission line 4 is connected to the homodyne detector 40 via the fiber coupler 30. The synchronous detection signal (FIG. 1.E) output from the homodyne detector 40 is A / D converted by the digital processor 50, the attenuation constant of the ring-down pulse light is calculated, and the absorption characteristic of the sample is measured. .

  On the other hand, the first optical transmission line 4 is provided with an optical directional coupler 8, and a first optical switch element 12 is connected to the branch terminal. The first optical switch element 12 is an element that selectively branches the pulse light branched from the first optical transmission path 4 to n + 1 optical transmission paths having different reference optical path lengths. Optical transmission lines 200, 201, 202,..., 20n are provided which are made of optical fibers wound around n + 1 piezo tube scanners 15. These transmission lines have an optical path length that effectively changes due to the piezoelectric effect of electrical signals. It is. Specifically, the amplitude was set to 2 mm. A set of these n + 1 optical transmission lines is the second optical transmission line 20. Each of these second optical transmission lines 20 is input to the fiber coupler 30 via the second optical switch element 13.

  As the wavelength-tunable femtosecond soliton pulse light source 1, a femtosecond pulse laser using an Er-doped fiber is used. The pulse width is preferably 10 to 500 fs. In this embodiment, the pulse width is 100 fs and the pulse interval is 20 ns (pulse frequency 50 MHz). Further, an optical soliton having a wavelength of 1630 nm was used. Of course, it may be pulsed light of about 1 ps.

The reference optical path lengths of the optical transmission paths 200, 201,..., 20n are effective optical path lengths L ₁ + L ₂ of the first optical transmission path 4 between the optical directional coupler 8 and the fiber coupler 30, and When the length of one round is L, it is set to L ₁ + L ₂ + nL (n = 0, 1, 2,...). That is, the reference optical path lengths of adjacent transmission paths differ by the length L of one round of the optical fiber 6.

  In the configuration of FIG. 2 described above, when the transmitted light is detected, the first optical switch element 12 and the second optical switch element 13 are switched and the 0th optical transmission line 200 is selected. When synchronously detecting the ring-down pulse light once, the first optical transmission line 201 is selected. Similarly, in the case where n-time ring-down pulse light is synchronously detected, the nth optical transmission line 20n is selected. In this way, by switching and controlling each optical transmission line and effectively vibrating the optical path length of each optical transmission line, FIG. A synchronous detection waveform such as E can be obtained. Then, the intensity ratio of each set is calculated from the synchronous detection waveform. Then, the attenuation constant is measured from the amplitude attenuation characteristic with respect to each number of ringdowns. In the present embodiment, high sensitivity detection can be performed based on the same principle as that of homodyne detection. In fact, it was possible to obtain 500 times the sensitivity (minimum detection amount 1/500) as compared with the ATR method.

[Modification]
In the above-described embodiment, the first optical transmission line 4 is shared with the transmission line for incident pulsed light to the optical fiber 6 and the output transmission line for extracting the ring-down pulsed light from the optical fiber 6. However, a separate output optical transmission line is provided, and a ring-down pulse is connected to the output optical transmission line via an optical directional coupler at a position different from the position of the optical switch 6 of the optical fiber 6. Light may be output. Both the optical transmission path for output and the second optical transmission path 20 are input to the homodyne detector 40. The optical path length of the ring-down pulse light to the homodyne detector 40 and the second optical transmission path 20 The optical path length of the reference light propagating through the light beam may be set so as to have the above relationship.

In the above embodiment, the optical fiber 6 is configured in a loop shape, but it may be a straight line or a curved line. That is, a linear optical fiber is coupled to the first optical transmission line 4 by the optical switch 5. Then, both ends of the optical fiber are used as mirror surfaces to reflect light. Even in this case, a ring-down pulse light traveling back and forth through a linear or curved optical fiber that is not a loop is output to the outside, and the optical path length of the pulse light is effectively equal to the second optical transmission path. Synchronous detection may be performed with reference light propagating through.
The principle is the same as above.

In the present invention, the pulse width of the pulsed light is not reduced so much by increasing the length of the optical fiber 6.
As a light source to which the present invention is applied, a supercontinuum light source may be used. In order to obtain information on a specific wavelength from the supercontinuum light source, each interference waveform (pulse) may be subjected to fast Fourier transform, the intensity of the specific wavelength of each pulse is obtained, and the attenuation time constant may be obtained.

  The apparatus of Example 2 is shown in FIG. Elements having the same functions as those in FIG. 2 showing the apparatus of Example 1 are given the same reference numerals. The fs laser light source 1 having a wavelength of 1.55 μm was used, and the length of the loop-shaped optical fiber (corresponding to the cavity) corresponding to the optical fiber 6 of FIG. 2 was 3 m. The laser from the fs laser light source 1 is branched by the optical directional coupler 8 into the first optical transmission line 4 and the optical fiber delay line 210-213 side. The laser branched to the optical fiber delay line side is input to the drive mirror 53 that is displaced by a minute width via the optical directional coupler 54, and the laser reflected by the mirror 53 passes through the optical directional coupler 54. The light enters the optical fiber delay line 210-213. The drive mirror 53 has the same function as the piezo tube scanner 15 of the first embodiment, and vibrates the optical path length of the laser with a minute width. The optical fiber delay lines 210-213 are made of fibers having optical path lengths of 3, 30, 60, and 180 m, respectively. The optical switches 12 and 13 are switched to select any one optical fiber delay line. The interference waveform between the laser propagated through the optical fiber delay lines 210 to 213 and the ring-down light output from the optical fiber 6 is detected by the balanced homodyne detector 40. Although the optical switch 5 is used for coupling to the loop-shaped optical fiber 6 in FIG. 2, a variable ratio coupler 51 is used in this embodiment. For the sample measurement head portion 52, a product prepared by polishing to near the optical fiber core was used. The head 52 makes it possible to measure the absorption of the sample by the interaction between the evanescent wave of the light propagating in the optical fiber 6 or the near-field light and the sample.

FIG. 4 shows interference waveforms in a 3 m optical fiber delay line and a 30 m optical fiber delay line. Pure water and methanol were used as samples. It can be seen that even at 3 m and 30 m, methanol has a lower interference intensity and absorbs more than pure water. FIG. 5 shows the change in the absorptance derived from the interference intensity by changing the methanol concentration with the optical fiber delay line as 30 m. The sample methanol was measured by dropping 10 μL of the methanol onto the head part 52. Since the effective optical path length of the head unit 52 used here was about 1 mm, it was calculated that the absorption coefficient such as methanol was 2 cm ⁻¹ (at a wavelength of 1.55 μm) and the sample was a very small amount of 2 fL. It can be seen that sufficient measurement is possible with the sample.

  Moreover, since methanol has a relatively large absorption coefficient, a large absorption coefficient could be measured even with a 30 m optical fiber delay line having an optical path length 10 times that of the optical fiber corresponding to the cavity. In the case of methanol, 3 ppm, 2 fL of ethanol can be measured with an optical fiber delay line of about 2 km from the calculation.

Furthermore, since a large absorbance is not observed even in the case of a sample having a small absorption coefficient, it is necessary to perform measurement with a longer optical fiber delay line in order to increase sensitivity.
By switching such optical fiber delay lines with different optical path lengths, it is possible to measure from a high concentration sample to a very low concentration sample. If a wavelength tunable soliton pulse light source is used, the absorption coefficient can be measured. Can be measured over a wavelength range of about 1 μm to about 2 μm.

Embodiment 2
If the optical path between the two mirrors forming the cavity is L, the difference between the optical path lengths of each pulsed ring-down light is 2L, and if the speed of light is c, the time interval between adjacent ring-down lights is 2 L / c. That is, FIG. As shown in A, the light pulse output from the cavity includes transmitted light S ₁ , S ₂ , S ₃ , S ₄ ,... That are not reflected between the mirrors. Further, after 2 L / c of the transmitted light S _1, the one-time ring-down light L _{11 that} is reflected and output once by the two mirrors is further reflected by the two mirrors after the time 2 L / c. The ring-down light L _{12 that} is reflected twice and output as light is output in sequence at a time interval of 2 L / c. The same applies to the transmitted light S ₂ , S ₃ , and S _4. By adjusting the optical path L and the pulse interval T _p between the two mirrors, a set of ring-down light that follows one transmitted light becomes the next. It is easy to avoid overlapping with the transmitted light.

On the other hand, the reference light (reference) is detected through a separate path and homodyne detection with a pulse of transmitted light that follows a set of ring-down light that is signal light. Assuming that the optical path length of the reference light is completely matched with the optical path length of the transmitted light, the homodyne detection of the reference light and the signal light is the same as the reference light pulse. The detection output at this time is A. Next, assuming that the optical path length of the reference light is completely matched with the optical path length of the ring-down light once, the homodyne detection of the reference light and the signal light is also the same as the pulse of the reference light. At this time, the detection output is αA (0 <α <1). Similarly, assuming that the optical path length of the reference light is completely matched with the optical path length of the n-down ring-down light, the homodyne detection of the reference light and the signal light is also the same as the pulse of the reference light, and the detection output is α ⁿ A (0 <α <1).

  However, it is not always easy to make the optical path length of the reference light completely coincide with the optical path length of the transmitted light and each ring-down light. This is a difficulty that comes from using signal light with a short pulse width. Therefore, a movable mirror that can vibrate is provided in the optical path of the reference light. Then, due to the vibration of the movable mirror, adjacent pulses of the reference light pass through different optical path lengths, and the pulse interval of the reference light reflected by the mirror becomes longer or shorter than before reflection. At this time, if the amplitude a of the movable mirror is made shorter than the optical path L between the two mirrors forming the cavity, the time interval 2L / c between the adjacent ring-down lights is caused by a change in the pulse interval of the reference light. The change 2a / c in the pulse time can be reduced. In this way, the optical path length of the reference light obtained by branching the signal light having a desired pulse frequency (pulse interval) is vibrated by a movable mirror that can be oscillated, so that the optical path length of the reference light is changed between the transmitted light and the ring-down light each time It can easily be achieved to match the optical path length almost perfectly.

In this embodiment, the principle is the same as that shown in the first embodiment, as shown in FIG. In the following description, interference between two pulse trains in the embodiment of the present application will be described by exemplifying specific numerical values. The signal light has a pulse width of 100 fs (1 × 10 ⁻¹³ seconds), a pulse interval of 2 × 10 ⁻⁸ seconds (pulse frequency 50 MHz), and the vibration frequency of the oscillating mirror placed in the middle of the reference light is 20 Hz. deep. Further, it is assumed that the optical path length of the reference light coincides with the optical path length of the signal light at the vibration center of the mirror. In addition, the mirror vibrates ± 4 mm from the position.

Assuming that the position of the mirror is x (mm) with the vibration center at time t seconds, and t = 0 and x = 0, x = 4 sin 40πt. The mirror speed v (mm / sec) is v = 160πcos 40πt, where t = 0 and v = 500 mm / sec. Then, since the mirror moves 1 × 10 ⁻⁸ m while two pulses of the reference light having a pulse interval of 2 × 10 ⁻⁸ seconds arrive, the optical path difference becomes 2 × 10 ⁻⁸ m. This means that the pulse interval of the reference light reflected by the mirror is longer or shorter by 6.67 × 10 ⁻¹⁷ seconds than before reflection.

Then, for example, in the signal light and the reflected reference light, the 0th pulse is changed to FIG. Like B, the pulse width is shifted by 1 × 10 ⁻¹³ seconds, the time of the pulse on the reference light side after reflection is earlier, and the pulse interval on the reference light side after reflection is longer than the pulse interval of the signal light If 6.67 × 10 ⁻¹⁷ seconds is longer, 1 × 10 ⁻¹³ seconds ÷ 6.67 × 10 ⁻¹⁷ seconds = 1500, so the reference light side pulse and signal light reflected after the 1500th pulse are reflected. The pulses coincide, and at the 3000th pulse, the signal light and the reference light after reflection are shifted by exactly 1 × 10 ⁻¹³ seconds of the pulse width, and the time of the pulse on the reference light side after reflection is delayed. Since the pulse interval of the signal light is 2 × 10 ⁻⁸ seconds, the time interval between the 0th pulse and the 3000th pulse is 6 × 10 ⁻⁵ seconds.

When such signal light and reflected reference light interfere with each other and homodyne detection is performed, FIG. An output such as C is obtained. FIG. Interference occurs between the signal light and the reflected reference light during the 3000 pulses in the state B. The 0th pulse is just 0, the 1500th pulse is the maximum, and the 3000th pulse is just 0. 2999 peaks are obtained below the C envelope (indicated by the dotted line). This envelope and the 2999 peaks below it exist for 6 × 10 ⁻⁵ seconds and do not exist until 0.05 seconds after the mirror returns to its original position. That is, the envelope and 3000 peaks below it exist for 6 × 10 ⁻⁵ seconds at 0.05 second intervals, and do not exist at other times.

When viewed macroscopically, very sharp peaks with a width of 6 × 10 ⁻⁵ seconds will occur at 0.05 second intervals (FIG. 1.D). The pulse waveform is as shown in FIG. Of course, it does not have a square wave shape like B. In addition, FIG. Regarding the difference in interference between the B 1500th pulse and the preceding and following pulses, for example, 18.4 waves exist in one pulse at a wavelength of 1.63 μm, and the phase difference is as small as 2π / 80. Then, interference between a plurality of sets of signal light near the peak of the envelope and the reference light after reflection produces an intensity substantially equal to the intensity of the peak of the envelope, so the phase of the pulse between the signal light and the reflected light of the reference light There is no need to adjust so that they are perfectly matched. Further, the position of the vibration center of the mirror does not need to be adjusted so that the optical path length of the reference light reflected there completely coincides with the optical path length of the signal light. That is, it is only necessary that the optical path length of the reflected reference light has a position that completely matches the optical path length of the signal light in the range in which the mirror vibrates. Therefore, homodyne detection with the signal light can be easily performed by guiding a pulse train having a sufficiently high pulse frequency with respect to the vibration frequency and amplitude of the mirror to the reference light path.

In this way, since it is sufficient that the optical path lengths are almost the same, even if the vibrating mirror is moved in the direction of the optical path, there are only a plurality of places where the optical path lengths coincide with each other during vibration. Homodyne detection can be easily performed. Then, the vibration center of the vibrating mirror is shown in FIG. A position where the optical path length of the reference light matches the optical path length of the transmitted light S ₁ , S ₂ , S ₃ , S ₄ ,. A position where the optical path length of the reference light coincides with the optical path length of the one-time ring-down light L ₁₁ , L ₂₁ , L ₃₁ , L ₄₁ ,. A position where the optical path length of the reference light coincides with the optical path length of the two ring-down lights L ₁₂ , L ₂₂ , L ₃₂ , L ₄₂ ,. If it is moved to the position where the optical path length of the reference light coincides with the optical path length of the n ring-down lights L _1n , L _2n , L _3n , L _4n,. Like E, interference with transmitted light, interference with ring-down light once, interference with ring-down light twice, interference with ring-down light n times, each having a plurality of widths 6 × 10 ⁻ It can be detected as a very sharp peak of ⁵ seconds.

  FIG. 6 is a block diagram showing a configuration of a cavity ring-down spectroscopic analyzer 300 according to a specific embodiment of the present invention. The cavity ring-down spectroscopic analysis apparatus 300 includes a tunable femtosecond soliton pulse light source 301, a half-wave plate 302, a spherical lens 303a, a polarization maintaining fiber 304, a spherical lens 303b, a beam splitter (light branching means) 305, a polarized beam. A splitter 306, a quarter-wave plate 307, and a galvanometer mirror 308 that changes the position and vibrates are provided. The signal light (signal light) passes through the wavelength-variable femtosecond soliton pulse light source 301, the half-wave plate 302, the spherical lens 303a, the polarization maintaining fiber 304, and the spherical lens 303b, and forms a cavity from the beam splitter 305. The light is guided to the mirrors 311 and 312, and the ring-down light reaches the biconvex lens 321. At the same time, the reference light (reference light) reaches the beam splitter 305 in the same manner, and then passes through the polarizing beam splitter 306 and the quarter wavelength plate 307 and is reflected by a galvano mirror 308 that can change its position and can vibrate. Thus, the light reaches the biconvex lens 322 from the polarization beam splitter 306 via a quarter-wave plate. The signal light that reaches the biconvex lens 321 passes through the optical fiber 331, and the reference light that reaches the biconvex lens 321 passes through the optical fiber 332, both of which are guided to the fiber coupler 330 and balanced via the optical fibers 331 ′ and 332 ′. Guided to mold detector 340 (homodyne detection means). The electric signal output from the balanced detector 340 is analyzed by the A / D conversion and digital processing device 350.

  As the wavelength-tunable femtosecond soliton pulse light source 301 of the cavity ring-down spectroscopic analyzer 300, a femtosecond pulse laser using an Er-doped fiber is used. The pulse width is preferably 10 to 500 fs. In this embodiment, the pulse width is 100 fs and the pulse interval is 20 ns (pulse frequency 50 MHz). Further, an optical soliton having a wavelength of 1630 nm was used.

  The galvanometer mirror 308 vibrates in the up and down direction in the drawing with a total width of 8 mm at 20 Hz. In addition, the galvanometer mirror 308 can be moved 300 mm downward in the drawing.

  The cavity region formed by the high reflection mirrors 311 and 312 has a cavity length of 35 mm and 46 mm. High reflection mirrors 311 and 312 both have a reflectivity of 99.8% or more. With the configuration of FIG. A waveform such as E is detected, and the intensity ratio of each set is calculated. Further, the time difference between the time when the transmitted light of the signal light is output and the time when each n times of ring-down light is output is calculated separately. Thus, the intensity of each n-th ringdown light with respect to the transmitted light was plotted against the time difference between the transmitted light and each n-th ringdown light, and the absorption coefficient of the sample placed in the cavity was obtained from the time constant.

FIG. 8 shows actual waveform data obtained with the configuration of FIG. This waveform is an interference waveform before the envelope is taken, but the response speed of the photodetector is as slow as 1 kHz. A pulse train with a pulse width of 6 × 10 ^-5 seconds at E is observed as one large interference pulse waveform. Furthermore, if a balanced homodyne detector using a photodetector with a fast response is used, FIG. Measured with a waveform like E.

  In the present embodiment, high sensitivity detection can be performed based on the same principle as that of homodyne detection. In fact, it was possible to obtain 500 times the sensitivity (minimum detection amount 1/500) as compared with the ATR method.

FIG. 7 is a configuration diagram showing three methods for arranging a substrate with a sample attached to the surface in the cavity region.
First, FIG. Like A, infrared light is transmitted through the substrate (Sub) on which the sample (Obj) is attached. In this case, it is necessary to use a substrate having high transparency with respect to infrared light to be used, and it is necessary to easily adjust the absorption coefficient and the substrate thickness.
Second, FIG. Like B, infrared light is reflected at the interface with the substrate (Sub) on which the sample (Obj) is attached. In this case, it is necessary to use a substrate having a high reflectance with respect to infrared light to be used. There is no need to adjust the substrate thickness.
Third, FIG. Like C, the sample is attached to the side surface A corresponding to the bottom of the isosceles trapezoidal bottom of the prismatic prism (Priz) whose base is an isosceles trapezoid, and corresponds to the side of the bottom of the isosceles trapezoid. Infrared light is introduced from the side B to be reflected and reflected at the interface between the side A of the prismatic prism and the sample attached thereto, and corresponds to the side of the bottom surface of the isosceles trapezoid that is paired with the side B It is made to permeate | transmit from the side C to do. This utilizes absorption of evanescent waves by the sample (Obj) in the vicinity of the reflection point. Furthermore, FIG. In C, the high reflection mirror 11 may be provided on the side surface B and the high reflection mirror 312 may be provided on the side surface C, and only the inside of the prism (Pliz) may be used as the cavity.

  As a light source to which the present invention is applied, supercontinuum light having a spectrum as shown in FIG. 9 may be used. FIG. 10 shows an interference waveform obtained using this light source and having the same configuration as that shown in FIG. In order to obtain an absorption coefficient of a specific wavelength from such an interference waveform, each interference waveform (pulse) may be subjected to fast Fourier transform, the intensity of the specific wavelength of each pulse is obtained, and the attenuation time constant may be obtained.

Embodiment 3
The present invention will be described based on specific examples, but the present invention is not limited to the following examples.

  As shown in FIG. 11, a first optical transmission line 412 for introducing light is optically coupled to a loop-shaped optical fiber 410 by a directional optical coupling element 436. A sample 440 is inserted into the transmission path of the loop-shaped optical fiber 410. The sample 440 is provided in a space between the end surfaces by cutting the optical fiber 410 so that the end surfaces face each other. The sample 440 is configured to pass light. Further, the optical fiber 410 is provided with an optical amplification element 415. The first optical transmission line 412 is composed of an optical fiber. A laser device 430 capable of outputting continuous laser light is connected to one end of the optical fiber, and the other end is received by the detection element 432 and the detection element 432. In order to prevent the ring-down pulse light from being attenuated, a processing device 434 is provided for controlling the amplification factor of the optical amplification element 415 and calculating the absorption characteristics of the sample from the amplification factor.

  The first optical transmission line 412 is provided with a directional optical coupling element 436 that optically couples the loop-shaped optical fiber 410 and the first optical transmission line 412. In addition, a second polarizer 421 is provided in front of the directional optical coupling element 436 on the laser light incident side, and a first polarizer 422 is provided in front of the detection element 432 on the laser light incident side. A Faraday rotator 423 serving as a polarization control element is provided in front of the directional optical coupling element 436 on the laser light incident side. The second polarizer 421 is, for example, an element that outputs only the S-polarized component. The Faraday rotator 423 is an element that applies a magnetic field in the transmission path direction according to a control signal from the processing device 434, rotates the polarization by 90 degrees, and changes the polarization from, for example, S polarization to P polarization. The directional optical coupling element 436 is an element that branches only the P-polarized light to the optical fiber 410 only in the traveling direction. The first polarizer 422 is an element that allows only light polarized in a predetermined direction, for example, P-polarized light to pass therethrough.

  The continuous laser light output from the laser device 430 enters the second polarizer 421, and only the S-polarized component is output to the first optical transmission line 412. The S-polarized continuous laser light passes through the Faraday rotator 423 to which no magnetic field is applied, and enters the directional optical coupling element 436. However, since the directional optical coupling element 436 branches only the P-polarized light to the optical fiber 410, no laser light is output to the optical fiber 410 in this state. The S-polarized continuous laser light that has passed through the directional optical coupling element 436 is incident on the first polarizer 422. Since the first polarizer 422 passes only the P-polarized light, the S-polarized continuous laser light is It does not enter the detection element 432. Therefore, the continuous laser light output from the laser device 430 does not enter the detection element 432 and does not prevent the detection element 432 from receiving the ring-down pulse light.

  When a pulse magnetic field is applied to the Faraday rotator 423 by the pulse control signal output from the processing device 434, the laser light passing through the Faraday rotator 423 during this application period has a 90-degree phase from S-polarized light to P-polarized light. Rotate. In other words, pulse P-polarized light is obtained at the output of the Faraday rotator 423, and is continuously S-polarized light outside that period. The pulsed P-polarized laser light is incident on the directional optical coupling element 436, is branched to the optical fiber 410, and circulates the optical fiber 410 clockwise in the drawing. Each time it circulates, light absorption occurs in the sample, and the amplitude of the pulsed P-polarized laser light decreases sequentially. That is, P-polarized ring-down pulsed light is obtained. Each time this ring-down pulse light circulates, a part of the light is branched to the first optical transmission line 412 side via the directional optical coupling element 436. Since it is P-polarized light, the light is partially branched to the first optical transmission line 412 side by the directional optical coupling element 436 and is incident on the first polarizer 422. Since the first polarizer 422 passes only the P-polarized light, the ring-down pulsed light is incident on the detection element 432.

  The processing device 434 controls the amplification factor of the optical amplification element 415 so that the amplitude of the ring-down pulse light detected by the detection element 432 is not attenuated. Therefore, when the absorption rate and the amplification factor of the sample are equal, the amount of attenuation in the sample 440 is amplified in advance by the amplification element, so that the pulsed light passing through the sample can be prevented from being attenuated. That is, the amplitude of the pulsed light train can be prevented from changing. At this time, it is sufficient that the amplification factor of the optical amplifying element 415 can follow the time variation of the absorption rate of the sample. By performing this measurement while changing the wavelength of the continuous laser light, the wavelength absorption characteristics of the sample can be obtained, and the atomic and molecular structure of the sample can be specified. In this case, since the absorption coefficient can be measured with the amplitude of the light incident on the sample being constant, the nonlinear effect can be eliminated, and the absorption coefficient with respect to the light intensity can be accurately measured. it can. In other words, if the above measurement is performed while changing the intensity of the laser beam, the nonlinear characteristic of the absorption coefficient of the sample can be obtained.

  In addition, since there is attenuation of the measurement system itself, in practice, the ring-down light of the pulsed P-polarized laser light when there is no sample is measured and the ring-down light is not attenuated in the same manner as above, By controlling the amplification factor of the amplification element, the attenuation amount of the measurement system can be measured from the amplification factor. By correcting the absorption rate of the sample measured as described above with the attenuation amount of the measurement system, it is possible to obtain the true absorption rate of the sample. Therefore, an extremely accurate absorption rate measurement is realized.

  The ring-down characteristic of the pulsed P-polarized laser light is measured with the horizontal axis as the number of ring-downs and the vertical axis as the amplitude of the ring-down pulsed light. In this characteristic, the amplification factor of the amplification element is controlled by a minute amount, and the amplification factor may be feedback-controlled so that the amplitude of the ring-down pulse light is not attenuated.

  In the above embodiment, the directional optical coupling element 436 is generally well known. With this directional coupling element, the ring-down pulse light of the laser light can be extracted from the optical fiber 410 to the detection element 432. Further, the intensity of light circulating through the optical fiber 410 can be adjusted by changing the coupling rate. Thereby, since the attenuation width of the light received by the detection element 432 can be adjusted, the attenuation coefficient can be measured with the same dynamic range, and the accuracy can be improved.

  In the above embodiment, the laser device 430 is a continuous wave laser, but this may be a pulsed laser, and the first polarizer 422, the second polarizer 421, and the Faraday rotator 423 may be eliminated. In this case, pulse laser light can be incident on the optical fiber 410 from the first optical transmission line 412 via the directional optical coupling element 436.

  In this embodiment, a semiconductor optical amplifier is provided in the optical fiber 6 in the apparatus shown in FIG. The configuration of the apparatus is shown in FIG. In FIG. 12, the configuration of the optical transmission path 412, the loop-shaped optical fiber 410, the insertion position of the sample 440, the detection element 432, and the processing device 434 is the same as that of the fourth embodiment. A pulse laser is used for the laser device 430, and the first polarizer 422, the second polarizer 421, and the Faraday rotator 423 are not used.

  In this embodiment, a semiconductor laser 430 having a wavelength of 1.55 μm was used as a pulse laser, and pulse oscillation was performed by current modulation. In order to acquire ring-down pulse light with high S / N, a balanced homodyne detector 440 was used. Therefore, unlike the device of the fourth embodiment, the pulse laser is split into two by the 1 × 2 optical directional coupler 408 as in the device of the second embodiment shown in FIG. 4 is introduced into the loop-shaped optical fiber 410 through the variable ratio coupler 450 through the first optical transmission line 404, while the other is introduced into the optical fiber delay line 460-463 as a reference signal for homodyne detection. .

  At the insertion position of the sample 440, a head 452 in which the side surface of the optical fiber is polished to the vicinity of the core and the evanescent wave or near-field light in the fiber interacts with the sample is inserted. The signal light that has circulated from the optical fiber 410 provided with the head 452 and interacted (absorbed) with the sample via the variable ratio coupler 450 is introduced into the 2 × 1 optical directional coupler 470. The reference signal passes through one of four types of optical fiber delay lines having different lengths through the 1 × 4 optical switch 472, and passes through the 4 × 1 optical switch 473 to the 2 × 1 optical directional coupler 470. Incident. The signal light and the reference light introduced into the optical directional coupler 470 interfere when the optical path length on the reference light side is close to an integral multiple of the loop optical path. Therefore, a minute displacement drive mirror 453 is inserted on the reference light side and the mirror 453 is driven, whereby an interference waveform is introduced into the homodyne detector and detection is performed with a high S / N. Other configurations are the same as those in FIG.

  An amplifying element 415 is inserted into the loop-shaped optical fiber 410, and a semiconductor amplifier (SOA) 415 is used in this embodiment. The amplifying element 415 is not limited to SOA, and an optical fiber amplifier using rare earth such as Er may be used. When a high output laser such as an fs laser is used, amplification is difficult with SOA, so an optical fiber amplifier is suitable.

  FIG. 13 shows the measurement results of the SOA amplification factor when the injection current to the SOA power supply is changed. As can be seen from FIG. 13, by using SOA, a signal can be amplified by about 16 dB. The signal intensity when the SOA amplification factor is 0 dB (amplification factor 1) and the optical path length of the optical fiber is changed to 30 m, 60 m, and 180 m is shown in FIG. Like A. FIG. A black circle in A is the interference intensity with respect to the optical path length when there is no sample in the head. It is greatly attenuated by the insertion loss of optical fiber heads, SOAs, optical connectors, etc. When 10 μL of 20% methanol is dropped onto the head in this state and the evanescent wave leaking from the light in the optical fiber is absorbed by methanol, the interference intensity is as shown in FIG. As indicated by the triangle A, the interference intensity is weakened by the amount absorbed. When the noise level of the homodyne detector is a dotted line level, the signal is buried in noise when the optical delay optical path length is 60 m or more, and high-sensitivity measurement becomes impossible.

  On the other hand, when the gain in the looped fiber is corrected by adjusting the amplification factor of the SOA, FIG. As shown in B, the interference intensity when there is no sample in the head changes from a triangle mark when the SOA amplification factor is 1 to a black circle, and is corrected so that the interference intensity does not change even if the optical path length of the optical delay line is increased. can do. When the sample is present in the head in this state, it is absorbed and the interference intensity is as shown in FIG. It looks like a triangular triangle. In this case, FIG. Unlike the triangle mark B, the interference intensity does not fall below the noise level even at 60 m or more. In the present embodiment, the optical path length of the longest optical delay line is 180 m, but measurement is possible even with an optical delay line of several kilometers or more, and a sample having a very small absorbance can be measured.

  Next, FIG. 15 shows an embodiment using a CW laser. With the same apparatus configuration as that of FIG. 12 shown in the fifth embodiment, the same measurement can be performed by changing only the laser apparatus to a CW oscillation semiconductor laser. The interference intensity when the optical delay path length is changed is adjusted appropriately in the same way as pulse oscillation, and the interference shown in Fig. 16 (a) drives the mirror regardless of the optical path length of 3m or 180m. Is observed by a homodyne detector. If there is a sample having absorption in the head portion in this state, an interference waveform as shown in FIG. 16B is obtained, and the absorption characteristics can be measured with high sensitivity from the attenuation rate of these interferences. In this case, the interference waveform that has been absorbed as shown in FIG. 16 (b) is appropriately increased in gain so as to have the same interference intensity as in FIG. 16 (a), and the absorption characteristic may be obtained from the gain at this time. .

Embodiment 3
Next, a seventh embodiment which is a specific embodiment of the present invention will be described. In FIG. 17, the configuration of the first optical transmission line 412, the loop-shaped optical fiber 410, the insertion position of the sample 440, the detection element 432, and the processing device 434 is the same as that of the fourth embodiment. A pulse laser is used for the laser device 430, and the first polarizer 422, the second polarizer 421, and the Faraday rotator 423 are not used.

  In this embodiment, a laser light input system and a laser light output system for the optical fiber 410 are separated. In the input system, a terminal 424 that does not transmit or reflect light is connected to the terminal of the first optical transmission line. As a newly provided laser beam output system, a second directional optical coupling element 437 optically coupled to the optical fiber 410, a second optical transmission line 413 coupled to the optical fiber 410 by the second directional optical coupling element 437, A termination terminal 425 is provided. The detection element 432 is connected to one end of the second optical transmission line 413.

  In this embodiment, the pulse laser beam output from the laser device 430 propagates through the first optical transmission line, branches to the optical fiber 410 via the directional optical coupling element 436, and circulates through the optical fiber 410. The ring-down pulse light circulating through the optical fiber 410 is incident on the detection element 432 via the second directional optical coupling element 437 and the second optical transmission path 413. The processing device 434 performs feedback control of the amplification factor of the optical amplification element 437 so that the amplitude of the pulsed laser light incident on the detection element 432 is not attenuated. Then, the absorption coefficient of the sample is calculated from the amplification factor of the amplification element. In this case, since the input system and the output system are separated, it is not necessary to use a polarizer or a polarizing directional coupling element for the second optical transmission line 413 that is the output system.

  In the above embodiment, the optical fiber 410 is configured in a loop shape, but it may be a straight line or a curved line as shown in FIG. That is, the linear optical fiber 490 is coupled to the first optical transmission line 412 by the directional optical coupling element 436. Then, both ends of the optical fiber 490 are mirror surfaces 442 and 443 so as to reflect light. Even in this case, pulsed light that travels back and forth through a linear or curved optical fiber 490 that is not a loop can be output to the detection element 432 side. At this time, only the ring-down pulse light propagating through the optical fiber 490 to the sample 440 side can be output to the first optical transmission line 412 and incident on the detection element 432 by the function of the directional optical coupling element 436.

As shown in FIG. 19, this embodiment excludes the first polarizer 422, the second polarizer 421, and the Faraday rotator 423 that is a polarization control element in the configuration of the seventh embodiment, and a directional optical coupling element 436. Instead, a piezo drive coupling rate variable coupler 436 as an optical coupling control element is provided. Other configurations are the same as those of the seventh embodiment. The piezo drive coupling rate variable coupler 436 is an element that controls the coupling rate using the electro-optic effect, and is an element that can control the optical coupling rate by applying a voltage. By applying a pulse voltage to the piezo drive coupling rate variable coupler 436 to increase the coupling rate in a pulse manner, pulsed light can be incident on the optical fiber 410. The fact that the ring-down characteristic of this pulsed light is detected by the light receiving element 432 is the same as in the seventh embodiment.
The optical coupling control element may be an optical switch instead of the piezo drive coupling rate variable coupler. In other words, even if the optical switch element can be connected and disconnected from the first optical transmission line 412 to the optical fiber 410 at high speed, pulsed light or step-reduced light can be incident on the optical fiber 410.

  In all the examples, the following examples can be considered. In a state where the sample 440 is not installed, the gain of the optical amplifying element is feedback-controlled so that the ring-down light is not attenuated. Next, the sample 440 is installed, the attenuation characteristic of the ring-down light is measured, and the attenuation constant is measured from this characteristic. In this case, since the attenuation due to the measurement system is excluded from the attenuation characteristic of the ring-down light, the absorption coefficient of only the sample can be accurately obtained.

  In the above-described embodiments of Embodiment 4 to Embodiment 10, the light propagating through the optical fibers 410 and 490 may be replaced with pulse light and may be light that decreases in steps. In this case, in the fourth embodiment, a continuous magnetic field is applied to the Faraday rotator 423 by a continuous control signal output from the processing device 434. As a result, the S-polarized continuous laser light output from the second polarizer 421 is converted to P-polarized light by the Faraday rotator 423. P-polarized laser light is incident on the optical fibers 410 and 490 via the directional optical coupling element 436. Next, the continuous control signal is interrupted, and the application of the magnetic field to the Faraday rotator 423 is stopped. Then, the output of the Faraday rotator 423 becomes S-polarized continuous laser light, and this light does not enter the optical fibers 410 and 490. Therefore, in the optical fibers 410 and 490, the amplitude of the P-polarized laser light decreases stepwise. This step-down light ring-down light is detected, and the amplification factor of the amplifying element 415 is feedback-controlled so that the light is not attenuated. Based on this amplification factor, the absorption rate of the sample can be measured.

  In the seventh and eighth embodiments, the laser device 430 may be a continuous laser, and a shutter may be provided in the first optical transmission line 412 so that the propagation of laser light is sharply blocked by this shutter. Further, the laser oscillation itself may be suddenly stopped. Even in this way, the laser light that decreases in steps can be propagated to the optical fibers 10 and 100. In the ninth embodiment, in the initial state, the coupling ratio between the first optical transmission line 412 and the optical fiber 410 is increased by the piezo drive coupling ratio variable coupler 436, and continuous laser light is incident on the optical fibers 410 and 490. Next, the piezo drive coupling rate variable coupler 436 is controlled to reduce the coupling rate between the first optical transmission line 412 and the optical fiber 410 stepwise. Then, the continuous laser light incident on the optical fibers 410 and 490 is blocked stepwise. The ring-down light of this step decreasing light is detected, and the amplification factor of the amplifying element 415 is feedback controlled so that the light is not attenuated. Based on this amplification factor, the absorption rate of the sample can be measured.

  Further, in all of the embodiments 4 to 10, an optical switch may be used instead of the directional optical coupling element 436. That is, the first optical transmission line 412 and the optical fibers 410 and 490 are coupled in the light propagation direction, the optical fibers 410 and 490 are closed, and the light propagated through the optical fibers 410 and 490 is the first. You may use the optical switch which can switch the mode propagated to the downstream of 1 optical transmission path 412. FIG. It is good also as an optical switch element which switches a switch terminal synchronizing with the period of ringdown pulse light.

  Further, the amplification factor is fixed to the amplification factor of the amplification element 415 when the sample 440 does not exist, and the absorption measurement of the sample 440 is measured from the attenuation light amount measured by the detection element 432 when the sample 440 exists. You can also.

  In Example 7 shown in FIG. 17, continuous laser light may be introduced into the optical fiber 410. When continuous laser light is used, the amplification factor of the amplification element 415 is small when the sample 440 is not present, and when the sample 440 is present, the amplification factor of the amplification element 415 is large due to light absorption by the amplification element 415. . The absorption characteristic of the sample 40 can be measured by the difference in amplification factor.

  As described above, when feedback control is performed so that the amplitude of the laser beam passing through the sample is not attenuated, the light absorption coefficient of the sample when the intensity of the laser beam is a predetermined value can be measured, and the measurement accuracy can be improved. Can be improved. Further, since the laser light is amplified to a level at which the loss in the optical fiber and the optical system can be ignored, the loss of only the sample can be accurately measured by improving the S / N ratio.

Embodiment 4
The present invention will be described based on specific examples, but the present invention is not limited to the following examples.

  As shown in FIG. 20, a first optical transmission line 512 for introducing light is optically coupled to a loop-shaped optical fiber 510 by a directional optical coupling element 536. A sample 540 is inserted in the transmission path of the loop-shaped optical fiber 510. The sample 540 is provided in the space between the end faces by cutting the optical fiber 510 so that the end faces face each other. The sample 540 is configured to pass light. The first optical transmission line 512 is composed of an optical fiber, and a broadband super continuum optical laser device 530 is connected to one end of the optical fiber, and the light receiving element 532 and the light receiving element 532 receive light at the other end. A processing device 534 for calculating an attenuation coefficient from the ring-down pulse light is provided. The processing device includes a light receiving element 532 and a processing device 534. The broadband super continuum laser device 530 is a device that outputs a broadband super continuum laser beam by using nonlinearity of the optical fiber or Raman amplification. The broadband super continuum laser light is well known, for example, by Norihiko Nishizawa, Toshio Goto, Solid State Physics Vol. 39 No. 10, (2004), pp 665-678.

  The first optical transmission line 512 is provided with a directional optical coupling element 536 that optically couples the loop-shaped optical fiber 510 and the first optical transmission line 512. The directional optical coupling element 536 branches the light propagated through the first optical transmission path 512 to the optical fiber 510 only in the traveling direction, and branches the light propagated through the optical fiber 510 to the first optical transmission path 512 only in the traveling direction. It is an element. When pulse laser light is introduced into the optical fiber 510 from the first optical transmission path 512, the pulse laser light circulates through the optical fiber 510, and each time it passes through the sample 540, its amplitude decreases sequentially, and ring-down pulse light. Is obtained. This ring-down pulse light is output from the optical fiber 510 to the first optical transmission line 512 via the directional optical coupling element 536.

  By measuring the ring-down attenuation coefficient of the ring-down pulse light with the light receiving element 532 and the processing device 534, the attenuation coefficient of the sample can be measured. By performing this measurement for each wavelength of the laser beam, the wavelength absorption characteristics of the sample can be obtained, and the atomic and molecular structure of the sample can be specified.

The relationship between the wavelength and propagation delay time per meter when the optical fiber 510 is a single mode high dispersion fiber is shown by SMF in FIG. When the wavelength changes from 1.33 μm to 1.94 μm, the propagation delay time changes by 13 ps / m. Accordingly, if the length of the optical fiber 510 is 1 km, the delay time changes by 13 ns when the wavelength changes from 1.33 μm to 1.94 μm. That is, when broadband supercontinuum light having a wavelength of 1.33 μm to 1.94 μm is used, the pulse width is increased by 13 ns for each ring-down pulse. For example, when a femtosecond laser beam (100 fs (1 × 10 ⁻¹³ sec)) is used as a pulse laser beam in a broadband super continuum with a wavelength of 1.33 μm to 1.94 μm, the width of the first ring-down pulse is 13 ns. The width of the second ring-down pulse is 26 ns, and the width of the third ring-down pulse is 39 ns. These ring-down pulses are illustrated in FIG.

  If the pulse period of the femtosecond laser light is 3 kHz, 100 ring-down pulses can be allowed during one period. Even if there are 100 ring-downs, the pulse width is 1.3 μs, and the interval between adjacent ring dumb pulses is 3.3 μs, so they do not overlap. A ring-down pulse as shown in FIG. 21 received by the light receiving element 32 is sampled at a minute time interval, and the value is temporarily stored. If it is not possible to sample the ring pulse for one pulse laser beam shown in FIG. 21 at a time, the pulse laser beam is repeated at 3 kHz, so that the ring-down pulse with the same waveform is repeated at 3 kHz. The repetitive waveform may be sampled to obtain a ring-down pulse waveform per cycle.

If the ring-down pulse per one-pulse laser beam shown in FIG. 21 is obtained, the time shown in FIG. 21 and the wavelength are in a fixed relationship, so that the wavelength can be determined from the time. For example, assume that the characteristic shown in FIG. 22 is a straight line, y = k (x−x ₀ ) + y ₀ . Where x is the wavelength, x ₀ is the center wavelength of this characteristic, 1.64 μm, y is the delay time, and y ₀ is the delay time at the wavelength x ₀ . Further, for the sake of simplicity, y ₀ = 0 is assumed. The waveform in FIG. 21 is represented by a delay time of 6.5 ns per Δx, where y ₀ = 0 and the maximum wavelength displacement from x ₀ is Δx (0.3 μm). Therefore, for one ring down, k = 6.5 ns / Δx = 21.67 ns / μm and y ₁ = k (x−x ₀ ). For the second ring-down pulse, y ₂ = 2k (x−x ₀ ), and for the third ring-down pulse, y ₃ = ₃ k (x−x ₀ ). However, y is a delay time based on the center of each ring-down pulse, that is, the time of each ring-down pulse at the center wavelength x ₀ .

As long obtained ring-down pulse waveform of FIG. 21, the time sequence y _1, y ₂ from the above _{relationship,} it is possible to determine the wavelength x corresponding to ... y _n. Then, with respect to the wavelength x, the time sequence y _1, y _2, ... from the damping characteristics of the pulse of the values in y _n, calculates the decay rate of the ring-down pulse alpha. If this is calculated | required regarding each wavelength x, the attenuation factor (alpha) (x) regarding a wavelength can be calculated | required. Actually, since the wavelength propagation delay time characteristic of FIG. 22 is not a straight line, the relationship between the time and the wavelength of FIG. 21 is obtained using this curve.

  As described above, if the length of the optical fiber 10 is 1 km, the pulse width of the ring-down pulse is at least 13 ns and 130 ns at the 10th ring-down pulse. Therefore, since the pulse width is sufficiently wide, waveform sampling is facilitated and the wavelength resolution is increased. Even if the length of the optical fiber 10 is 100 m, the pulse width of the ring-down pulse is 1.3 ns at the minimum and 13 ns at the 10th ring-down pulse, and the waveform of FIG. 21 can be sampled.

The repetition period of the pulse laser beam is related to the length of the optical fiber 10. If the total length is 1 km, a ring-down pulse is output every 3.3 μs. Therefore, if the repetition frequency of the pulse laser beam is 1 kHz (period 1 × 10 ⁻³ sec), 300 ring-downs are performed during one pulse period. Pulses can be tolerated. If the total length of the optical fiber 10 is 100 m, a ring-down pulse is output every 0.33 μs. Therefore, if 100 ring-down pulses are allowed during one pulse period, the pulse period is set to 30 kHz. There is a need.

  In addition, since the measurement system itself is attenuated, in reality, the ring-down characteristic of the pulsed laser when there is no sample is measured as the reference characteristic, and the ring-down of the pulsed laser when the sample is measured The absorption coefficient of the sample is measured using the attenuation characteristic of the deviation of the characteristic with respect to the reference characteristic. The absorption coefficient may be calculated from an exponential decay coefficient when the horizontal axis is the number of ringdowns and the vertical axis is the amplitude of the ringdown pulse. Further, the wavelength absorption characteristic can be obtained by changing the wavelength of the laser beam and measuring the attenuation coefficient of the ring-down characteristic in the same manner. Even if the absolute value of the absorption coefficient is unknown, the sample can be identified if the relative absorption characteristic is obtained as the wavelength characteristic.

  In Example 12 described above, the directional optical coupling element 536 is generally well known. With this directional coupling element, a ring-down pulse of laser light can be extracted to the light receiving element 532. Further, the intensity of light circulating through the optical fiber 510 can be adjusted by changing the coupling rate. Thereby, since the attenuation width of the light received by the light receiving element 532 can be adjusted, the attenuation coefficient can be measured with the same dynamic range, and the accuracy can be improved.

Next, a thirteenth embodiment which is a specific embodiment of the present invention will be described. In FIG. 23, the configuration of the first optical transmission line 512, the loop-shaped optical fiber 510, the insertion position of the sample 540, the laser device 530, the light receiving element 532, and the processing device 534 is the same as in the twelfth embodiment.
In the thirteenth embodiment, the laser light input system and the laser light output system for the optical fiber 510 are separated. The input system is substantially the same as that of the twelfth embodiment except that a terminal 524 that does not transmit or reflect light is used at the end of the first optical transmission line 512. As a newly provided laser beam output system, a second directional optical coupling element 537 optically coupled to the optical fiber 510, a second optical transmission line 513 coupled to the optical fiber 510 by the second directional optical coupling element 537, A termination terminal 525 is provided. A light receiving element 532 is connected to one end of the second optical transmission line 513.

  In this embodiment, the method for guiding the pulse laser beam to the optical fiber 510 is the same as that of the twelfth embodiment. The ring-down pulse light circulating through the optical fiber 510 is incident on the light receiving element 532 via the second directional optical coupling element 537 and the second optical transmission path 513. In this case, since the input system and the output system are separated, it is not necessary to use a polarizer or a polarizing directional coupling element for the second optical transmission line 513 that is the output system.

  In the thirteenth embodiment, the optical fiber 510 is configured in a loop shape. However, as shown in FIG. 24, it may be a straight line or a curved line. That is, the linear optical fiber 590 is coupled to the first optical transmission line 512 by the directional optical coupling element 536. Then, both ends of the optical fiber 590 are mirror surfaces 542 and 543 so as to reflect light. Even in this case, it is possible to output pulsed laser light that travels back and forth along a linear or curved optical fiber 590 that is not a loop to the light receiving element 532 side. At this time, only the ring-down pulse light propagating through the optical fiber 590 to the sample 540 side can be output to the first optical transmission line 512 and incident on the light receiving element 532 by the function of the directional optical coupling element 536. Other configurations are the same as those of the twelfth embodiment.

The present invention is suitable for analysis such as identification of a very thin film and a very small amount of sample, and is suitable for analysis of a small amount of plasma-treated substance or a very small amount of DNA.
In addition, the present invention is effective for spectroscopic analysis of biological materials such as liquids, gases, DNA, and proteins, organic materials, inorganic materials, and thin films that have low light absorption. Further, in one invention, the wavelength absorption characteristic can be obtained from the ring-down pulse waveform without changing the wavelength of the laser light source.

1: wavelength tunable femtosecond soliton pulse light source 2: half wave plate 3a: spherical lens 4: first optical transmission line 5: optical switch 6: optical fiber 7: optical amplifier 8: optical directional coupler 12: first Optical switch element 13: Second optical switch element 15: Piezo tube scanner 20: Second optical transmission line 200, 201,..., 20n ... Optical transmission line constituting second optical transmission line 30: Fiber coupler 40: Homodyne detector 50: Processing device 301: Wavelength variable femtosecond soliton pulse light source 302: 1/2 wavelength plate 303a, 303b: spherical lens 304: polarization maintaining fiber 305: beam splitter 306: polarization beam splitter 307: 1/4 wavelength plate 308: Galvano mirrors 311, 312: High reflection mirrors 321, 322: Biconvex lens 330: Fiber coupler 331, 32, 331 ', 332': Optical fiber 340: Balanced detector 350: A / D conversion and digital processing device 410, 490 ... Optical fiber 421 ... Second polarizer 422 ... First polarizer 423 ... Faraday rotator 412 ... First optical transmission line 413 ... Second optical transmission line 415 ... Optical amplification element 436, 437 ... Directional optical coupling element 510, 590 ... Optical fiber 521 ... Second polarizer 522 ... First polarizer 523 ... Faraday rotator 512 ... First optical transmission line 513 ... Second optical transmission line 536, 537 ... Directional optical coupling element

Claims (4)

The pulsed laser light is guided to the optical fiber that interacts with the sample whose light absorption characteristics are to be measured, and the ringdown pulse light due to the light absorption of the sample is output to the outside, and the absorption characteristics of the sample are measured from the attenuation characteristics of this ringdown pulse light In the spectroscopic method characterized by:
The pulse laser beam is a laser beam having a broad spectrum, the optical fiber is a strong dispersion optical fiber, the pulse width of the ring-down pulse light is sequentially increased, and the wavelength absorption is performed from the ring-down attenuation constant of the pulse train in the time train corresponding to the wavelength. A spectroscopic method characterized by measuring characteristics. In a spectroscopic device that measures the light absorption characteristics of a sample,
A strongly dispersed optical fiber that guides the laser pulse light to the sample whose light absorption characteristics are to be measured;
A laser device that generates laser pulse light having a wide spectral width;
The pulse width of the ring-down pulse light that circulates or reciprocates through the optical fiber is sequentially widened and output to the outside. In the attenuation characteristic of this ring-down pulse light, from the ring-down attenuation constant of the pulse train in the time train corresponding to the wavelength And a processing device for obtaining wavelength absorption characteristics.   A first optical transmission line that optically couples to the optical fiber, and a directional optical coupling element that optically couples the first optical transmission line to the optical fiber, and the laser device, etc. at one end of the first optical transmission line The spectroscopic device according to claim 2, wherein a light receiving element of the processing device that receives the ring-down pulse light is provided at an end.   The ring-down pulse light is output to a second optical transmission line different from the first optical transmission line by another optical coupling element disposed at a position different from the directional optical coupling element in the optical fiber, and the second The spectroscopic device according to claim 2, wherein a light receiving element of the processing device is connected to an optical transmission line.